When 1-alkene, particularly, ethylene and a monomer, such as alkyl(meth)acrylate, alkyl methacrylate ester or vinyl ester are copolymerized, a copolymer having different properties in comparison with that of a simple polyethylene may be formed. For example, an adhesive force and a low temperature toughness may be significantly improved as an amount of the monomer(s) increases. Accordingly, researches on copolymers formed from such 1-alkenes and other monomers have been actively conducted.
According to a conventional technique, a copolymer of ethylene or alkyl acrylate or a copolymer of ethylene and alkyl methacrylate is obtained by increasing an amount of a monomer under a condition of high temperature and high pressure. As a result, a tube-type reactor or an autoclave reactor which endures high temperature and high pressure has been used. For example, U.S. Pat. Nos. 2,953,551, 3,350,372, and 5,543,477 disclose that a reaction is progressed by adding concurrently ethylene and acryl based monomer in harsh a condition, such as at a pressure of 1000 bar or more and at a temperature of 100° C. or more, using an autoclave apparatus or a tube-type reactor. A polymer obtained from this reaction is a copolymer including 3˜35% of the acrylate based monomer that is a polar monomer based on the weight of ethylene.
In order to realize such condition of high temperature and high pressure, however, additional devices, such as a first compressor, a second compressor and a special reactor are required for safety of users. In addition, due to such harsh condition, to adjust the compositions in the copolymers to a desired direction, it is largely with many limitation requirements. That is, in a conventional method of preparing an ethylene based copolymer, even a capacity of a compressor used in the process and a post process after the reaction should be considered to control a melting temperature of the copolymer in the range of 50˜100° C.
In addition, the ethylene based copolymer prepared according to the conventional method includes only a small amount of a polar group. That is, the amount of a polar monomer is so small that polyethylene crystals remain in the copolymer, which is not desired for optical products, such as a transparent film. Although in ethylene vinyl acetate and ethylene methyl acrylate which are the conventional products, a melting temperature decrease as an amount of the ethylene monomer increases, and is not sufficient to develop amorphous resins in the conventional method. Thus, the polymers produced using the conventional tube-type reactor or autoclave at high temperature and high pressure are applicable only to products which are not largely affected from the opacity of the polymers.
Accordingly, there is a need to develop a new method of preparing a copolymer which is amorphous due to presence of a large amount of a monomer having a polar functional group therein, while avoiding the condition of high temperature and high pressure.
On the other hand, a method of preparing copolymer of olefin and vinyl monomer using a metallic complex compound catalyst has been known. Due to high affinity of a metal to oxygen, however, the method has a disadvantage that the metallic complex compound catalyst based on transition metal and lathanide based metal in the first development stage is easily contaminated by a functional group (C═O) of a polar vinyl monomer. Although it has been reported that the copolymer of alkyl acrylate and olefin may be prepared using a metallic complex compound catalyst based on some of transition metal in the later development stage, the copolymer still has the large amount of olefin.
On the other hand, in order to overcome the disadvantage of the polymerization method by the metallic complex compound catalyst, that the metallic complex compound catalyst is contaminated by oxygen existing in a polar group and an activity of the metallic complex compound catalyst thus decreases and that a polymer having a small amount of a polar group contained therein are obtained, a controlled radical polymerization has been proposed. When the copolymer of olefin and polar vinyl monomer is produced by ATRP (Atom Transfer Radical Polymerization) method, which is a representative polymerization method among the controlled radical polymerization methods, an amount of the polar vinyl monomer is larger than that of olefin, contrary to the polymerization method by the metallic complex compound catalyst. That is, a random copolymer in which an amount of olefin is controlled in some degrees according to conditions of copolymerization may be produced. However, when the copolymer is produced by ATRP method, it takes long time to obtain high molecular weight and the copolymer having the so small amount of olefin is obtained. In addition, the 1-alkene-alkyl(meth)acrylate copolymer produced by the ATRP method has the small amount of ethylene in the polymer chain so that the copolymer can be easily broken and thus film properties decrease.
Since 1-alkene-acrylate based copolymer has high transparence, the polymer may be used in optical products. To do this, a thermal resistance should be excellent so that there is no deformation occurred by heat created when process of preparation and optical instruments are operating. Accordingly, there is a need to develop a copolymer which can improve properties such as the thermal resistance.
On the other hand, JP publication 2-308803 (date of the public: Dec. 21, 1990) discloses a method of preparing a copolymer of ethylene-unsaturated carbonic acid in which a copolymer of ethylene-unsaturated carbonic acid ester is obtained by a polymerization of ethylene and unsaturated carbonic acid ester under presence of Lewis acid using a catalyst consisting mainly of a chronic compound and an organometallic compound and the resultant copolymer of ethylene-unsaturated carbonic acid ester is then placed in a heat process of 80˜450° C. Wherein an amount of the unsaturated carbonic acid ester used in preparing the copolymer of ethylene-unsaturated carbonic acid ester is 0.001-45 mol %.